The aim of this project is to characterize viruses of mycoplasmas, especially those 3 found in spiroplasmas. One such virus (SVC3), previously plaqued on indicator lawns of Spiroplasma citri, was propagated in indicator broth cultures and purified by density gradient centrifugations. It contains 5 structural proteins of aggregate molecular weight 393,000; and its nucleic acid is linear double-stranded DNA of molecular weight 14 x 10 to the sixth power daltons. The DNA can transfect indicator cells. Rabbit antiserum to the virus neutralizes and agglutinates virions. These properties are the same for virus isolated from either S. citri or from the suckling mouse cataract spiroplasma. Studies of absorption, growth characteristics, and other properties of the DNA, are in progress. BIBLIOGRAPHIC REFERENCES: Nugent, K.M., and Cole, R.M.: Characterization of Group H streptococcal temperata bacteriophage phi 227. J. Virol. 21: 1061-1073, 1977. Cole, R.M.: Spiroplasma viruses. In Dalton, A.J., Hagenau, F., and Maramorosch, K. (Eds.): Insect and Plant Viruses: An Atlas. New York, Academic Press, 1977, pp. 451-464.